This application is a 35 U.S.C. xc2xa7371 application of PCT/EP00/00858, filed on Feb. 3, 2000, and claims the benefit of priority to German Application No. 199 07 065.2, filed on Feb. 19, 1999.
The present invention relates to a method of checking an isocentre and a patient-positioning device of an ion beam therapy system that is operated especially with heavy ions.
Ion beam therapy systems are preferably used in the treatment of tumours. An advantage of such systems is that, on irradiation of a target object (target), the major portion of the energy of the ion beam is transferred to the target, while only a small amount of energy is transferred to healthy tissue. A relatively high dose of radiation can therefore be used to treat a patient. X-rays, on the other hand, transfer their energy equally to the target and to healthy tissue, so that for health reasons, for the protection of the patient, it is not possible to use a high dose of radiation.
There is known from U.S. Pat. No. 4,870,287, for example, an ion beam therapy system in which there are generated from a proton source proton beams of which the protons can be delivered to various treatment or irradiation sites by an acceleration device. Provided at each treatment site is a rotating cradle having a patient couch so that the patient can be irradiated with the proton beam at different angles of irradiation. While the patient is spatially located in a fixed position inside the rotating cradle, the rotating cradle revolves round the body of the patient in order to focus the treatment beams at various angles of irradiation onto the target located in the isocentre of the rotating cradle. The acceleration device comprises a combination of a linear accelerator (LINAC) and a so-called synchrotron ring.
In H. F. Weehuizen et al. CLOSED LOOP CONTROL OF A CYCLOTRON BEAM FOR PROTON THERAPY, KEK Proceedings 97-17, January 1998, a method of stabilising the proton beam in proton beam therapy systems is proposed in which the treatment beam is actively so controlled that it is located on the centre line of the corresponding beam delivery system at two measurement points spaced from each other in the longitudinal direction. The first measurement point is located between a pair of deflection magnets and is formed by a multi-wire ionisation chamber. Depending on the actual value of the beam position delivered from that multi-wire ionisation chamber relative to the centre point of the beam path, a PI control is generated by further deflection magnets arranged upstream from the first-mentioned pair of deflection magnets. The second measurement point is located just upstream of the isocentre and is formed by an ionisation chamber which is divided into four quadrants. Depending on the actual position value of that ionisation chamber, again PI control signals are generated, but those control signals are intended for the first-mentioned deflection magnets. Such a control arrangement is said to render possible both angle stability in terms of the centre line of the beam delivery system and lateral position stability of the proton beam.
When, however, heavy ion irradiation is carried out, that is to say irradiation with ions that are heavier than protons, large and heavy devices are necessary, with the result that there is a tendency to avoid the use of rotating cradles and instead move the patient or the patient couch. Corresponding therapy systems are described, for example, in E. Pedroni: Beam Delivery, Proc. 1st Int. Symposium on Hadrontherapy, Como, Italy, Oct. 18-21, 1993, page 434. Such systems are accordingly eccentric systems.
Since, however, mainly isocentric systems are preferred by oncologists, a heavy ion beam therapy system was proposed in which, although rotating cradles are used at the treatment sites, the radii of the rotating cradles can be reduced by virtue of the treatment beam delivered to each rotating cradle horizontally along its axis of rotation being so guided by means of suitable magnet and optics arrangements that, for the irradiation of a target, the beam is first of all directed away from the axis of rotation and later crosses the axis of rotation again in the isocentre. There is provided for the irradiation of the target a grid scanner, which comprises vertical deflection means and horizontal deflection means, each of which deflects the treatment beams perpendicular to the beam axis, with the result that an area surrounding the target is scanned by the treatment beams. Such a system thus essentially provides beam guidance in only one plane of the rotating cradle.
The irradiation by the grid scanner is carried out with the aid of radiation dose data that are calculated automatically by the supervisory control system of the ion beam therapy system according to the patient to be irradiated or treated.
Since a high level of operational safety and operational stability in terms of the treatment beam is always necessary in ion beam therapy systems, a monitoring device for monitoring the treatment beam delivered by the grid scanner is provided in the afore-described heavy ion beam therapy system. This monitoring device is arranged between the last deflection magnet of the above-mentioned magnet arrangement and the isocentre, and can comprise ionisation chambers for monitoring the particle flow and multi-wire chambers for monitoring the beam position and the beam width.
For safety reasons, various DIN standards have to be observed in the operation of medical electron accelerators. Those standards are concerned on the one hand with the inspection test, that is, the inspection of the readiness for operation, and on the other hand with the consistency test, that is, examination of operational stability, of the system. For ion beam therapy systems, especially for heavy ion beam therapy systems, safety standards of that kind developed specifically for such systems are not yet known, but there is still a need, in ion beam therapy systems too, for as high as possible a level of operational safety and operational stability.
The problem underlying the present invention is therefore to propose a method of checking an isocentre and a patient device of an ion beam therapy system in order to improve operational safety and operational stability, especially in respect of the checking of the medical device for positioning of the patient. The method shall at the same time be suitable especially for use with heavy ions.
The problem is solved in accordance with the present invention by a method of checking an isocentre and a patient-positioning device of a heavy ion beam therapy system comprising the steps of representing a centre point inside a spherical phantom by means of a metallic specimen body, rendering visible the centre point of the specimen body by several image-forming methods, aligning laser beams on the centre point of the specimen body, checking lines formed from the laser beams for horizontality and verticality measuring a spatial position of the centre point from a predetermined threshold, and inspecting the heavy ion beam therapy system. The dependent claims each define preferred and advantageous embodiments of the present invention.
According to the present invention, an ion beam therapy system is operated that comprises a grid scanner device, arranged in a beam guidance system, having vertical deflection means and horizontal deflection means for the vertical and horizontal deflection of a treatment beam perpendicular to its beam direction, with the result that the treatment beam is deflected by the grid scanner device onto an isocentre of the irradiation site, and a specific area surrounding the isocentre is scanned, wherein a check of the isocentre and a patient-positioning device is carried out and the patient-positioning device comprises a patient table rotatable about an axis of rotation of the table. For the checking, a target point within a spherical phantom is represented by means of a special specimen body and the centre point of the specimen body is visibly represented by means of several image-forming methods. On departure of the measurement results in respect of the spatial position under the image-forming methods from a predetermined intervention threshold, the ion beam therapy system is subjected to an inspection and maintenance.
Preferably, the accuracy of the stereotactic determination of coordinates of a target point should be checked by means of a CT (computer tomography) or MR procedure (nuclear spin resonance procedure), since the accuracy of the stereotactic image formation is a crucial factor for the overall accuracy of the irradiation. For that purpose, it is possible for any desired target point to be represented inside a spherical phantom by means of a special specimen body, the centre point of which can be visibly represented by means of the image-forming method. The spherical phantom is inserted into the stereotactic frame so that the centre point becomes an unknown target point. The stereotactic coordinates are then ascertained one after the other in terms of time using the applied X-ray, CT or MR method, wherein in the tomographic method the layer spacing should be 1 mm. Since the X-ray method is accurate to {fraction (1/10)} mm, the accuracy of the determination of the target point by CT and MR can be ascertained by comparison with the X-ray method, that is to say, the radial spacing between the position of the target point ascertained by X-ray image and the position ascertained by the CT or MR method is checked. The radial spacing should not exceed 1.5 mm. For the purpose of checking consistency, it is sufficient for this test to be carried out annually.
As a further checking aspect it is proposed that the accuracy of the position of the isocentre between the axis of rotation of the patient couch and the central beam of the grid scanner be checked since the isocentre, defined as the point of intersection between the axis of rotation of the patient couch and the central beam of the grid scanner is the connecting element in the positioning between planning and irradiation. A check for consistency should be carried out prior to each block of irradiation procedures.
In order to check the isocentre in relation to the axis of rotation of the patient couch, a metallic specimen body (2-3 mm in diameter) is introduced, with the aid of lasers, into the nominal isocentre, that is to say into the nominal axis of rotation of the patient couch. The specimen body is maintained in fixed position by means of a plumb bob, which is centred precisely on the centre point above the specimen body. On rotation of the patient couch about the axis of rotation, the extent to which the specimen body moves in relation to the plumb bob is ascertained. This procedure is carried out at at least three different levels of the patient couch, with a maximum displaceability of the patient couch up or down of 15 cm, for example at the level of the isocentre 10 and with a minimum of 15 cm distance above and below. The maximum departure allowable is 1.0 mm in the direction of the beam and only 0.5 mm perpendicular to the direction of the beam. Variations that are in the beam direction are less critical, since dose distributions in the patient are not affected by such variations.
In order to check the isocentre in relation to the central beam 11, the position of the isocentre is, by definition, fixed on the axis of rotation of the patient couch below the plane for the straight-ahead beam, and is ascertained relative to wall markers by means of an optical measurement system. Checking the position of the specimen body relative to the central beam 11 is carried out by a film measurement, a verification film being irradiated, downstream of the specimen body viewed in the direction of the beam, with a (undeflected) central beam, the half-value width of which is greater than the diameter of the specimen body, with the result that the position of the specimen body is projected on the verification film relative to the central beam. In this case the intervention threshold is at a maximum 25% departure from the half-value width of the primary beam.
In addition, the accuracy of the laser alignment on the isocentre 10 must be checked, since the lasers mark out the isocentre. In this procedure, following positioning of the specimen body in the isocentre, the lasers are aligned onto the centre point of the specimen body by means of optical measurement, and the departure of the laser lines from the horizontal and vertical are checked, the maximum departure allowed in each case being 1 mm. In order to check consistency, the image of the lasers on the opposite-lying walls or on the floor is marked out and then used as a reference value.
A further checking aspect is concerned with the accuracy of the alignment of the X-ray tubes and of the target cross on the opposite-lying recording stations, since the X-ray method represents an additional procedure for marking out the isocentre. After positioning the specimen body in the isocentre by means of optical measurement, that is to say using lasers, X-ray images are taken in the three spatial directions and the spacing between the projected image of the specimen body and the target cross on the X-ray image is ascertained. The image of the specimen body should be projected precisely onto the image of the target cross, so that the maximum permissible spacing between the projected image of the specimen body and the target cross is 1 mm.
Owing to the isocentric irradiation of the patients, it is also necessary for the accuracy of the display of the angular scale of the isocentric rotation of the patient couch to be checked, and this can be carried out analogously to the provisions of DIN 6847-5, point 12.2.4. The maximum tolerable inaccuracy is 1xc2x0.
The spatial stability of the isocentric rotation of the patient couch should likewise be checked, since a corresponding stability is a prerequisite of the definition of the isocentre. This check can be carried out analogously to DIN 6847-5, point 14.2, the intervention threshold being an inaccuracy of 1 mm.
It is finally also proposed that the accuracy of the placing and positioning of the patient is checked, since accurate patient positioning is a prerequisite for proper irradiation for the tumour in question. In that connection, for the inspection test and to check consistency (prior to each block of irradiation procedures) of the therapy system, the unknown stereotactic coordinates of the centre point of a specimen body, which has been fixed within the stereotactic base ring, are ascertained as the target point and, with the aid of the stereotactic targeting device and by means of transverse movement of the patient couch, the centre point is brought into the isocentre. In that position, X-ray images are taken in the three spatial directions and the spacing of the position of the specimen body from the target cross is ascertained on the three images. The maximum radial spacing allowed between the centre point of the specimen body and the isocentre is 1.5 mm. Otherwise an appropriate correction of the placing of the patient is necessary.
It is proposed especially that the calculated radiation dose values be checked for a plurality of measurement points of the phantom, adequate accuracy of the calculation of the radiation dose data being inferred when the average discrepancy between the calculated and measured values of the radiation dose for all measurement points does not exceed a predetermined first tolerance value and when for each individual measurement point the discrepancy between the calculated and the measured radiation dose for that measurement point does not exceed a predetermined second tolerance value. The first tolerance value is xc2x15% and the second tolerance value xc2x17%.
In order to check for a correct transfer of the geometric structures at the treatment site and to check the planning parameters of an image-forming device of the ion beam therapy system up to the time of positioning, a digital reconstruction, especially an X-ray reconstruction, can be calculated from the phantom, which reconstruction is compared with an X-ray image generated from the phantom in order to ascertain a possible discrepancy.
The present invention renders possible a clear improvement in the operational stability and operational safety of an ion beam therapy system and defines a checking plan having particular checking aspects that can be performed in the sense of an inspection test and/or a consistency test of the ion beam therapy system. This relates especially to irradiation planning, in the course of which radiation dose data are automatically calculated in the ion beam therapy system according to the patient to be irradiated or treated.